1. Field of the Invention
The present invention relates to a method for controlling the dewatering of sludge which comprises the step of adding an optimum amount of an organic polyelectrolyte coagulant to sludge produced in facilitier for the treatment of sewage, human excreta or industrial waste water and the like to efficiently dewater the sludge, and it also relates to an apparatus for controlling the dewatering of sludge.
2. Description of the Prior Art
In general, for example, in a dewatering process for sludge produced in a sewage treatment plant, an organic polyelectrolyte coagulant (hereinafter referred to as "polyelectrolyte coagulant" or simply "coagulant") is added to the sludge, and then dewatered cake is formed by a dewatering device. Conventional techniques for adding the coagulant are described in Japanese Patent Publication No. 31932/1982 and Japanese Patent Application Laid-open No. 25598/1985.
In a method for adding the coagulant which is described in Japanese Patent Publication No. 31932/1982, the concentration of the raw sludge is first measured, and the quantity of the sludge to be fed to a dewatering device and the amount of the coagulant to be added are then controlled on the basis of the measured concentration of the raw sludge.
In another method for adding the coagulant which is described in Japanese Patent Application Laid-open No. 25598/1985, the capillary suction time (hereinafter referred to simply as "CST"), defined below, of the coagulant-treated sludge containing the added coagulant is first measured, and an optimum coagulant dosage level at which the CST is minimized is determined based on the relation between the amount of the coagulant added and the CST of the coagulant-treated sludge. The actual coagulant dosage level is decided by multiplying the optimum coagulant dosage level by a factor.
CST is defined as the length of time required for a liquid to be absorbed by a fixed area of capillary material via capillary suction.
In the above-mentioned former method for measuring the concentration of the raw sludge, the raw slurry is introduced into the coagulant addition step via a thickening tank, and therefore the concentration of the sludge changes with the passage of the time, so that the necessary amount of the coagulant to be added varies. However, since the amount of the coagulant has been already determined on the basis of the concentration of the raw sludge, the amount of the coagulant cannot be properly controlled at times.
On the other hand, in the latter method for deciding the amount of the coagulant by measuring the CST of the coagulant-treated sludge, the CST tends to decrease with the increase of the coagulant amount, but it increases when the amount of the coagulant is in excess of a certain level. The CST being low means that sludge has grown to larger flocs. Therefore, if the amount of the coagulant is adjusted so that the CST may be minimized, it is sure that the sludge can be efficiently dewatered.
In this method, the CST is measured in the sludge containing the added coagulant, but it is very difficult to collect small representative samples of the coagulant-treated sludge due to the uneven distribution of flocs in said sludge. Thus, the flock amount is different each time the coagulated sludge is sampled, so that the measured values of the CST fluctuate and thus are unreliable, with the result that it is hard to properly control the amount of the coagulant.
In accordance with this method, a test was made in a sewage treatment plant by the present inventors. In this test, the estimated proper amount of the coagulant was added to a raw sludge (where the CST would be minimized) and the excessive amount of the coagulant was added to the raw sludge (where the CST would fairly increase). Afterward, each coagulant-treated sludge was sampled several times, and the CST (seconds) was then measured. As a result, with regard to the samples containing the proper amount of the coagulant, the measured CST values, for example, were 7.6, 9.9, 7.8, 9.7 and 8.3 seconds, and with regard to the samples containing the excessive amount of the coagulant, the measured CST values, for example, were 21.2, 13.0, 13.6, 19.7 and 19.8 seconds.
In both the cases, the measured CST values fluctuated greatly.
Incidentally, the above-mentioned measurement of the CST was made by the use of the undermentioned measuring device.
In addition to the above-mentioned methods, there have been suggested a dewatering control method utilizing a colloid charge amount (Japanese Patent Application Laid-open No. 200899/1986) and another control method utilizing an anion degree (Japanese Patent Application Laid-open No. 132599/1987). In the dewatering control method utilizing the colloid charge amount, the colloid charge amount of the sludge is first measured, and from the thus measured value, indications of the concentration and the characteristics of the sludge are determined. On the basis of these indications, conditions such as the sludge feed flow rate, the coagulant dosage level, the stirring strength of a mixing tank, the dewatering time and dewatering pressure are most properly controlled. In the control method utilizing the anion degree, the amount of crude suspended solids in the sludge, the anion degree of the sludge, and the cation degree of the coagulant are measured, and the dosage level of the coagulant is shown as a linear function of these factors to determine an optimum coagulant dosage level.
However, in order to measure various factors such as the colloid charge amount of the sludge, the amount of the crude suspended solids, the anion degree of the sludge and the cation degree of the coagulant, expert knowledge of physics and chemistry and experience are required, and measuring these factors is complex and time-consuming. For example, in the case where sludge from primary settling tanks, a excess activated sludge, and digested sludge are mixed and treated in a sewage treatment plant, the mixing ratio of these sludges changes continually. Therefore, if the dosage level of the coagulant is controlled in accordance with these factors determined for a sample sludge which does not necessarily represent the actual sludge, no good control can be exercised and so it is very difficult to steadily and efficiently dewater the sludge in the real sewage treatment plant.